1. Technical Field
This invention relates generally to an apparatus for the modulation of shear forces applied to a skin surface. More particularly, the instant invention relates to a multi-layer system of materials which, when applied directly to a skin surface, modulates shear forces which would otherwise be applied thereto and thereby increases or decreases such forces as desired.
2. History of Related Art
Many of the tasks undertaken on a daily basis require the human body to be physically contacted by various surfaces. When such contact takes on a repetitive nature, the skin draws on its natural resources to thicken and eventually, over a period of time, a callous may be produced. However, some activities are so abrasive, or conducted over such a short period of time, that these natural defenses have insufficient time to develop. In such cases, protective clothing such as shoes or gloves may be worn. However, such measures may also be insufficient to prevent the formation of blisters, ulcers, etc. In addition, bony prominences beneath the skin and other conditions lend themselves to perforation or ulceration of the outer skin layers when constantly contacted by coverings which are supposed to protect these areas, such as mattresses, mattress covers and bed sheets. In every case, it is the shear force applied to the skin surface which results in surface degradation. This problem makes itself known in the form of bed sores, ulcerations, bunions, blisters, and other physical manifestations symptomatic of excessive shear force, caused by excessive friction or reduced synthetic surface compliance, applied to the skin surface.
On the other hand, it is at times desirable to increase the magnitude of such shear forces by providing increased friction between the skin surface and whatever it contacts. A good example of this circumstance exists in the area of track and field competition, wherein sprinters require the maximum amount of friction between their forefoot and the insole of the running shoe to gain the maximum amount of acceleration in the shortest possible time. Other examples of demanding increased friction include the use of pine tar for application to batter's gloves, or other substances, as used in golf, and tennis, or any time that an athlete desires to gain greater control over an implement having a grip.
Several attempts have been made to address the problem of shear force reduction, most of these being in the area of reducing shear force, or its practical elimination. One example is found in U.S. Pat. No. 5,586,398, issued to Carlson, which discloses a shoe having a friction management interface built into the insole. The design is intended to reduce friction during heel strike, and increase friction on push-off. The amount of friction between the user's foot and the shoe is controlled by interaction between the sock surrounding the foot and an insert. However, this reference does not teach the use of shear force modulation through the use of friction management with directional control, or the use of materials which create a varying time/displacement response to shear forces.
U.S. Pat. No. 5,154,682, issued to Kellerman, describes a low-friction shoe insert made of polyethylene, which is bonded to the insole. However, in every embodiment taught by this reference, direct application of the foot sole onto slippery polyethylene is required.
A different approach to shear force reduction is disclosed by U.S. Pat. No. 2,254,883, issued to Boyle, which speaks to a protective pad having an adhesive strip onto which are applied three sheets of thin, smooth material. The outer layer of smooth material is applied against the skin, while the adhesive strip serves to hold the multi-layer arrangement in place. However, this device does not demonstrate the controlled use of friction or compliance to modulate shear forces with respect to direction, time, or magnitude.
A similar attempt is revealed in U.S. patent application Ser. Nos. 4,572,174 and 5,019,064, issued to Eilander et al., which describe a bed pad structure having a stationery lower layer bonded to a slippery top layer, with a fluid lubricant inserted therebetween. The slippery layer is porous, and is to be placed in contact with the skin. However, this device does not teach the use of a slippery interface between two high-friction surfaces, with the controlled modulation of shear forces.
One of the areas exhibiting a long-felt need for a shear force modulation system is that of insoles. Currently, over-the-counter insoles are made from a variety of single, double and triple layer combinations of viscoelastic materials. For the most part, competitors in the market attempt to separate themselves from rivals by unusual color and material combinations, as well as other marketing gimmicks, as opposed to effective material combinations which provide results due to their physical interaction. Available insoles help to reduce peak focal pressures measured between the foot and the insole, but none has been developed that modulates shear forces.
Therefore, what is needed is a shear force modulation system which provides precise control over shear forces applied to the skin, including time-response, quantity or magnitude of shear force, and directional control of shear force. In addition, such a device would be further enhanced by acting to reduce peak pressures (i.e., those forces applied in a vertical or normal direction to the skin surface, such as the impact forces on a foot engaged in walking or running).